Cation and anion exchange resins have been widely used to separate ions from a solution. However, hitherto known ion exchange resins could not be suitably used for separation of a particular cation or anion from a solution containing various kinds of cations and anions because of their weak selective adsorptivity.
As cation exchange resins capable of exhibiting selective adsorptivity, there have been used chelate resins composed of polymers introduced with chelate-forming groups, such as iminodiacetic and dithiocarbamic groups. Examples of such chelate resins include Dowex A-1 (trade name by Dow Chemical Ind.), Diaion CR-10 (trade name by Mitsubishi Chemical Ind.) and Unicellex UR-10 (trade name by Unitika Ltd.). Those chelate resins, however, exhibit selective adsorptivity to only divalent or more multivalent cations of, e.g., iron, copper, mercury, etc. They exhibit virtually no selectivity for monovalent cations of lithium, etc., divalent cations of alkaline earth metals, uranyl ion, and the like. On the other hand, no anion exchange resins have been put to practical use which are capable of selectively adsorbing a particular anion.
It is known that hydroxides of multivalent metals, such as titanium, iron, aluminium and zirconium, are capable of exchanging a particular anion or cation selectively. Taking advantage of this property, such hydroxides have been employed as effective ion exchangers. For example, selective adsorption properties of zirconium hydroxide to various anions and cations, in particular, to arsenite are described in Anal. Chem., 39, 2086 (1963). Selective adsorption properties of aluminium hydroxide to phosphate and arsenate ions are described in J. Radioanal. Chem., 4, 289 (1970). Commercial applications of such metal hydroxides, however, encounter the problem that they could hardly be applied to a column as they usually exist in the form of powder. In view of this, it has been proposed to use metal hydroxides shaped to spheric or microspheric forms. In one proposed method, powders of a metal hydroxide is admixed with a binder component, such as polyacrylonitriles, polyvinyl alcohols, polyacrylamides and polyacrylhydrazides, and then shaped to desired forms. For instance, J. Nacl. Sci. Tech., 14 (1), 811 describes shaped titanium hydroxide particles capable of selectively adsorbing uranium ions, in which a polyacrylamide is used as a binder. This kind of shaped products, however, have not been put to practical use due to various disadvantages. For example, they have only insufficient mechanical strengths and therefore do not withstand repeated uses. In another method so far proposed, a porous carrier, such as active carbon and organic polymers (e.g., MR type anion exchange resins of macro porous type) is dipped into an aqueous solution of a multivalent metal salt, removed out of the solution and then subjected to a treatment with an alkaline agent so as to deposit a metal hydroxide on the porous carrier. In U.S. Pat. No. 4,116,856, for instance, is described a process for adsorbing lithium ions selectively, using an adsorbent consisting of a MR type (macro porous type) anion exchange resin on which aluminium hydroxide is supported. However, adsorbents produced by the above method have not been put to practical use since they carry metal hydroxides only in amounts insufficient to attain a satisfactory ion exchange capacity.
It is therefore desired to develop unexpensive ion exchange resins capable of efficiently separating or removing metal ions contained, in particular, in water in trace amounts.